Whenever imaging sensors are used for quantitative analysis of incident radiation, precise control of the exposure time, i.e. the time during which the sensor is exposed to the incident radiation, is of essential importance. Either the exposure of the sensor must be as even as possible over its surface, or the inhomogeneities caused by the shutter must be known so precisely that subsequent calibration can be carried out.
In many sectors in which high speeds are not required, mechanical shutters are used. These however do not have arbitrarily short shutter times or arbitrarily high repeat rates, and their life is limited because of the mechanical wear.
Electro-optical shutter elements, such as for example acoustic-optical modulators, indeed have significantly faster time behavior but as attachments with separate control, they are not compact and therefore not suitable for all applications. The same applies to controllable image amplifier tubes.
New developments in the sector of imaging sensors however allow integration of the shutter into the sensor itself in the form of an “electronic” shutter: the radiation is no longer physically isolated from the sensor surface, but the signal is either recorded or rejected at the sensor itself.
Such an electronic shutter circuit cannot however be used for every type of imaging sensor. For CCD-based (CCD=Charge-Coupled Device) sensors for example, such a mechanism cannot be used. Local intermediate stores help but these only have finite storage capacity and also impose a loss of sensitivity.
Concepts on the basis of so-called active pixel sensors (APS) however are widely used. Depending on implementation, such an electronic shutter can be switched with very high cycle rates and very good homogeneity over the sensitive area of the corresponding sensor.
However, all systems currently used for implementing an electronic shutter, using the mechanical or electro-optical concepts specified above, share the common feature that the sensor is completely insensitive outside the opening time of the shutter. The shutter therefore leads to a sensor dead time. This is always a problem for time-continuous measurement, i.e. if images must be recorded in direct succession without dead time and with defined exposure time.
A shutter mechanism may however assume a further function beyond control of the exposure time. It can be used to prevent any signals which are received during the read process from disrupting the signal evaluation and falsifying the signals.
The signal detected by the actual sensor during the exposure time must be extracted from the sensor for processing, then amplified and digitized. Depending on the required precision, correspondingly longer processing times are required. If new signals are received during processing of the signals already present, the signal amplitude is falsified. The nature and extent of the falsification to be expected greatly depend on the sensor used.
For CCDs for example, the working cycle is divided into exposure time and transfer time. During the exposure time, charges generated by incident photons are integrated. During the transfer time, the signal charge is transferred to the read amplifier and amplified there. For time-critical applications, the CCD is read in parallel columns, i.e. each CCD column has its own amplifier channel, whereby the read-out speed can be multiplied approximately by the number of columns. Despite this, additional signals reaching the sensor during the charge transfer are not assigned correctly either in time or in position. These are so-called out-of-time events (OOT).
The abovementioned long signal processing times mean that the total read time is determined not only by the pure charge transfer, but quite significantly by the signal processing. Therefore attempts are made to decouple the transfer time from the signal processing time by creating an intermediate store (frame store) by doubling the number of pixels. There is now a sensor region and a frame store region. The signal charge is (quickly) transferred from the sensor region to the frame store region, and from there can be shifted (slowly) to the amplifiers arranged at the matrix end and amplified while another charge is integrated in the sensor region.
This complex measure indeed significantly reduces the number of OOTs but cannot suppress these completely because of the finite transfer time between sensor and frame store region. However in this case too, the integration time cannot be selected freely since the integration time must be at least as long as the read-out time.
Implementation of an electronic shutter in the CCD is not possible for design reasons. An external shutter can completely suppress any OOTs occurring, but again leads to a dead time.
As already stated, in a CCD the necessary transfer of the signal charge—and indeed of the entire collected charge in the sensor region—to the read node can, in the least favorable case, take place over the entire sensor (and frame store) length. This constitutes a substantial disadvantage of the CCD. A true window mode, in the sense of a rapid non-selectable access to the region of interest (ROI), is not therefore possible. This is the great advantage of a ‘true’ active pixel detector in which the signal charge is collected and amplified at the point of generation. Here there is no transfer of signal charges, and by corresponding connection and control of the pixels (image points), arbitrary regions can be selected and read with high repeat rate. One example for the implantation of this concept is a sensor matrix consisting of DEPFETs (Depleted Field Effect Transistor). Depending on the size of the ROI, the read speed can be multiplied locally relative to the entire matrix. The use of active pixels sensors however has disadvantages. These include, in particular in DEPFETs, erroneous signal detection due to the permanent sensitivity (see below) and the so-called “rolling shutter” effects.
These are provoked by the temporal offset on reading of different lines (and hence their integration time) and—in particular with rapidly moving objects—can lead to artefacts and image distortion.
Since the amplifying electronics integrated in the pixel lie within the sensor region, signal charges received during processing have a different effect than in the CCD. As described, in the CCD such events are incorrectly assigned in place and time. In the DEPFET, the charge carriers received at an arbitrary time during the read cycle are incorrectly weighted and thus falsify the detected signal amplitude. The DEPFET determines the signal by forming the difference between currents in the transistor of the active pixel before (signal current) and after a deletion pulse (reference current).
If the signal charges reach the internal gate of the DEPFET after deletion, the value of the initial current is falsified, which in the extreme case can even lead to paradoxical “negative” signal amplitudes.
Even more problematic for use however are charges which reach the internal gate of the DEPFET before deletion, since these are incompletely amplified. Such events are highly problematical in particular for spectroscopic applications, since the falsified signal amplitudes appear as an irreducible background in the spectrum.
Such signal falsifications are called “misfits” in the jargon. By their nature, they occur above all in temporally uncorrelated radiation, e.g. on astronomic observations or optical imaging, since here the reading of the sensor cannot be synchronized with the incidence of the radiation. The proportion of misfits to total events here corresponds to the ratio between the signaling processing time and the integration time. Applications in which the signal rate is high are therefore disproportionately affected by the problem, since firstly the total number of “misfit” events rises with the signal rate, and secondly the high signal rate requires an increase in the image rate, i.e. a shortening of the integration time, with otherwise unchanged signal processing time.
Accelerating the read-out by parallelization, e.g. by reading several lines of a matrix simultaneously, also aggravates the problem since the proportion of misfits increases further in proportion to the number of pixels read simultaneously. The extreme case, sensible from the viewpoint of an experimenter, of a hybrid DEPFET pixel sensor read fully in parallel is therefore the least favorable from the aspect of spectral usability of data.
This problem is caused by the permanent sensitivity of the pixel even during the read-out phase. One solution therefore is to switch the pixel insensitive during reading. The concept of the DEPFET allows integration of a conventional electronic shutter. EP 1 873 834 B1 for example proposes a DEPFET structure in which the detector can be switched insensitive during a definable time window, in that the incident electrons are extracted by the deletion contact. An additional electrode surrounding the internal gate of the DEPFET here prevents extraction of the electrons already stored there. This electronic shutter not only opens the possibility of controlling the exposure of the sensor with precision in the microsecond range. In addition, the DEPFET pixel can also be switched during reading to be insensitive to interference signals such as scatter light, thermally generated electrons or even signal electrons which would lead to misfits.
However this option is associated with a dead time of the total sensor which corresponds to the total read-out time. All signal electrons received while the shutter is closed are irrevocably lost. Line by line switching of the shutter indeed reduces the dead time to the read time of a line, but the property of the global shutter is lost as a result.
Some applications, e.g. polarimetry, impose further requirements for the electronic shutter, as well as a purely screening effect. One frequently used technique of polarimetry is based on detection of the so-called Stokes parameter. Here separate images are recorded while polarization filters are in different positions (typically 4). Since the polarization signal consists of the difference between images from two filter positions, the unpolarized proportion of the light disappears insofar as it does not change while the two images are being recorded. Often however the light is unavoidably disrupted on the path between source and detector. In astronomical observations, it is falsified by turbulence in the upper atmosphere layer. For polarimetry for example, in particular fluctuations in the unpolarized part with frequencies about or above the read-out rate are problematical since the unpolarized part is greater by many orders of magnitude than the polarized part. To suppress the effect of such fluctuations, the polarization plane should be changed and the respective associated image recorded at time intervals as short as possible.
For these studies, usually CCD-based sensors are used. Some instruments use frame-store CCDs which are operated with as high an image rate as possible. For each setting of the polarizer, a separate image is recorded by the CCD. The disadvantage of frame-store CCDs is that the polarization cannot be changed more quickly than the read-out rate of the CCDs. Also OOT-induced errors during shifting into the frame store must be corrected iteratively. Alternatively, special CCDs are used in which lines (typically 4) are covered strip by strip, wherein the lines collect the images of the individual polarizer settings (usually 4). Each polarizer setting is here assigned to a line and the signals of several cycles are cumulated in the respective line before the frame is read. The disadvantage of CCDs with covered columns is that a significant part of the quantum efficiency is lost, and during read-out the same errors occur as for frame-store CCDs as long as the CCD is not shaded otherwise (e.g. mechanically). Also both methods are associated with significant dead time and the maximum image rate remains coupled to the read-out speed.
The disadvantages of known detector arrangements therefore comprise the dead times in which the incident radiation is not detected, or alternatively the artefacts generated in the signal by the permanent sensitivity.